Present day integrated circuits require an increasing number of lines connections by which the integrated circuit communicates with the outside world. Physically an integrated circuit has a number of metallic pads around the periphery of the integrated circuit chips; the pads are connected to the rest of the integrated circuit. In use the integrated circuit chip is protected by a package which contains a number of metallic connections, typically pins, which electrically connect the integrated circuit to the outside world. The connections or pins are connected to the pads by metallic wires.
Many of these connections transmit the integrated circuit's internally generated signals to the outside world. These output connections require driver, or buffer, circuits in the integrated circuit. These driver circuits generate the required voltages and currents for the signals to be properly propagated to the outside world.
In the process of manufacturing integrated circuits, there is typically a step in which the individual integrated circuits which have been manufactured on a single semiconductor substrate, called a wafer, are tested to determine whether each of the integrated circuits function properly or not before the integrated circuits are packaged. In this step, the wafer sort operation, each integrated circuit is tested by a number of probes which each contact the input/output pads of the integrated circuit. The other end of the probes are connected to a computer which, under the control of a test program, transmits test pattern signals to the integrated circuit and collects the responses for analysis. The proper signal responses determine that the integrated circuit is functional or not.
A problem arises, though, with these large number of output connections. Necessarily these driver circuits generate large amounts of surge currents as the driver circuits quickly switch back and forth from one logic state to another (logic 1 to logic 0 and vice versa). The problem during wafer sort is that the long conducting lines of the probes to the computer create electrical signal reflections and other noises. These erroneous signals are aggravated by the large surge currents created by the driver circuits to cause the testing computer to misread and not correctly analyze the integrated circuit under scrutiny.
The present invention is directed toward solving or substantially mitigating this problem by providing for a driver circuit which can operate in two modes. In the first mode, the driver circuit of the present invention operates normally, i.e., with large initial surge currents and quickly changing voltage levels to maintain the high speed operation of the integrated circuit. In the second mode, the driver circuit behaves as a weak driver so that during wafer sort the problems caused by large surge currents from a typical driver circuit are substantially reduced.